©2014 Society of Economic Geologists, Inc. Economic Geology, v. 109, pp. 795–810
CRYPTIC VARIATION OF CHROMITE CHEMISTRY, PLATINUM GROUP ELEMENT AND PLATINUM GROUP MINERAL DISTRIBUTION IN THE UG-2 CHROMITITE: AN EXAMPLE FROM THE KAREE MINE, WESTERN BUSHVELD COMPLEX, SOUTH AFRICA Malte Junge,† Thomas Oberthür, and Frank Melcher* Federal Institute for Geosciences and Natural Resources (BGR), Stilleweg 2, D-30655 Hannover, Germany
Abstract The UG-2 chromitite of the Bushveld Complex in South Africa contains the world’s largest resources of platinum group elements (PGEs). However, only limited work has been conducted on the fine-scale chemical variation of chromite, and the distribution of PGEs and platinum group minerals (PGMs) within the UG-2. In the present study, one drill core (155 cm long) covering the sequence of the UG-2 chromitite at the Karee mine in the western Bushveld Complex was studied in detail. Geochemically, Pt correlates well with Pd, but not with Ni, Cu, and S, and Pt and Pd show high concentrations at the bottom and in the center of the main UG-2 layer. The mineral chemistry of chromite is characterized by distinct cryptic variation, leading to the division of the UG-2 sequence into 10 distinct sublayers. From bottom to top (with exception of the basal sublayer), each sublayer is defined by an upward decrease of Mg# combined with increases of Cr# and TiO2. It is proposed that these trends reflect magmatic differentiation of individual chromitite entities, which coalesced to form a massive chromitite seam. Sulfides (pentlandite, chalcopyrite) interstitial to chromite grains are rare, and associated PGM grains are mainly Pt-Fe alloy, laurite, and cooperite-braggite. The PGEs are bimodally distributed in the UG-2. Platinum and the IPGE (Os, Ir, Ru) are dominantly present as discrete PGMs, whereas large proportions of Pd and Rh are hosted in the crystal lattice of pentlandite. Palladium and rhodium contents of pentlandite reach a maxima of 2.2 wt % Pd and 3.0 wt % Rh. The present data suggest that a succession of chromitite sublayers formed due to magmatic differentiation and segregated sequentially on top of each other, finally forming a massive chromitite seam. Contemporaneously with chromite segregation, PGE-rich magmatic sulfides segregated with chromite grains, but large proportions of these sulfides were subsequently removed mainly by reaction of sulfide with chromite, thereby upgrading the PGE contents of the remaining sulfides. Upon cooling, the PGE-bearing sulfide droplets recrystallized, leading to a bimodal distribution of the PGEs in the UG-2. The bulk of the Pd and Rh is hosted in pentlandite, whereas Pt and the IPGEs, apparently incompatible in the remaining sulfides, formed a distinct suite of discrete PGMs.
Introduction The Bushveld Complex in South Africa (Fig. 1) is the largest layered mafic-ultramafic intrusion on Earth, with an estimated areal extent of 66,000 km², and has been dated to ca. 2050 Ma (Wilson, 1998; Scoates and Friedman, 2008). It contains the largest resources of platinum group elements (PGEs) on Earth and hosts economically important reserves of Cr and V (Vermaak, 1995). The formation of the layered sequence hosting the PGE-, Cr-, and V-rich layers still remains enigmatic and different models exist. Maier et al. (2013) argued that the cumulate rocks within the Bushveld Complex were generated by crystal sorting of slurries deposited on the sloping floor of subsiding magma chambers. Within the Bushveld Complex, PGE mineralization is mainly confined to a layered sequence of mafic-ultramafic rocks referred to as the Rustenburg Layered Suite, which is usually subdivided into the Marginal, Lower, Critical, Main, and Upper zones (Hall, 1932). The economic PGE-bearing orebodies within the Bushveld Complex are the Merensky Reef, the UG-2 chromitite, and the Platreef. Regional studies indicate that the Merensky Reef and the UG-2 are laterally continuous over hundreds of kilometers (e.g., Von Gruenewaldt et al., 1986; Hatton and Von Gruenewaldt, 1987; Scoon and Teigler, 1994). † Corresponding
author: e-mail,
[email protected] *Present address: Chair of Geology and Economic Geology, Montanuniversitaet Leoben, Peter Tunnerstraße 5, 8700 Leoben, Austria.
The UG-2 chromitite is the largest resource of PGEs on Earth (Vermaak, 1995) and nowadays represents the predominant source of PGEs from the Bushveld Complex. It occurs in the Upper group of the Critical zone, between 15 and 400 m below the Merensky Reef, whereby the smallest vertical separation is in the western and the greatest in the eastern Bushveld (Lee, 1996). The UG-2 chromitite has a general thickness of about 1 m, varying between 0.4 and 2.5 m (Schouwstra et al., 2000). The UG-2 layer is mainly a massive chromitite, consisting of 60 to 90 vol % of chromite with an average Cr/ Fe ratio between 1.26 and 1.4, and 43.5% Cr2O3 (Lee, 1996). Locally, the orebody consists of a number of smaller subsidiary seams that may bifurcate and merge with the main seam (e.g., Davey, 1992; Penberthy and Merkle, 1999; Maier and Barnes, 2008; Voordouw et al., 2009, 2010). Generally, the footwall of the UG-2 consists of a pegmatoidal feldspathic pyroxenite, or anorthosite, and the hanging wall of a pyroxenite (Mondal and Mathez, 2007). The PGE mineralization is generally limited to the UG-2 chromitite seam itself (Hiemstra, 1985, 1986; Von Gruenewaldt and Worst, 1986; Kinnaird, 2005; Kuhlmann et al., 2006; Maier and Barnes, 2008; Voordouw et al., 2009; Cawthorn, 2011). PGE contents are up to 10 ppm Σ[PGE + Au] and Pt/Pd ratios close to unity. Copper and Ni contents are generally below 0.05%, and S is generally below 0.1% (Von Gruenewaldt et al., 1986; Lee, 1996; Cawthorn, 2011). Accordingly, the amount of accessory base metal sulfides is low. In the present study, one drill core transecting the UG-2 chromitite of the Karee mine (LONMIN) in the western
0361-0128/14/4211/795-16 795
Submitted: June 11, 2013 Accepted: October 1, 2013
796
SCIENTIFIC COMMUNICATIONS Younger Sedimentary Cover & Intrusions
0
25
50
75
100
km
Granophyres + Granites Bushveld Rustenburg Layered Suite Complex Rooiberg Group Transvaal Pretoria Group Supergroup Chuniseport Group Archaean Basement Pre-Transvaal Formations PGE-reefs
Mokopane
f Burgersfort
Nietverdiend
Stoffberg Rustenburg Pretoria Pretoria
Belfast
Rustenburg
26°S
Karee Mine 26°
27°
Johannesburg 28°
29°
30°
Fig. 1. Geologic map of the Bushveld Complex, after Barnes and Maier (2002).
Bushveld Complex (Fig. 1) was studied in detail, using modern geochemical and mineralogical methods. The work centers on unraveling the fine-scale variation within the UG-2 chromitite regarding the distribution of PGEs (whole-rock data and concentrations in sulfides), platinum group minerals (PGMs), and the compositional variation of chromite. The study shows that the UG-2 chromitite is composed of a number of chemically distinct chromitite sublayers. Various geochemical and mineralogical trends are established that provide new insights into the relationship of chromitite and PGE mineralization. Previous Work In the Bushveld Complex, the correlation between chromitite and PGEs is exceptionally well expressed, as all the chromitite layers have elevated levels of PGEs, and even the thinnest chromitite stringers tend to contain elevated PGE grades (Maier et al., 2013). The origin of the PGE enrichment in chromitite layers remains controversial partly because many of the most PGE-enriched seams tend to be very S poor (commonly
